1. Field of the Invention
The present invention relates to a surface-emitting semiconductor laser having a laser emitting region at an upper surface and a manufacturing method thereof, and particularly relates to a surface-emitting semiconductor laser capable of being applied to applications where light output is necessary to be stable in the polarization direction and a manufacturing method thereof.
2. Description of the Related Art
The surface-emitting semiconductor laser is different from an edge-emitting laser in related art, which emits light in a direction orthogonal to a substrate and is capable of arranging many elements in a two-dimensional array state on the same substrate. Therefore, attentions is given to the surface-emitting laser as a light source for a digital copy machine or for a digital printer in recent years.
In this type of surface-emitting semiconductor laser in related art, a pair of multilayer film reflectors is formed on a semiconductor substrate and an active layer to be a light-emitting region is included between the pair of the multilayer film reflectors. In one of the multilayer film reflector, a current confinement layer having a structure in which a current injection region is narrowed is provided for increasing current injection efficiency to the active layer and for reducing threshold current. An n-side electrode is provided on the lower surface side and a p-side electrode is provided on the upper surface side respectively, and a light-emitting exit for emitting laser light is provided in the p-side electrode. In the surface-emitting semiconductor laser, electric current is injected into the active layer after being confined by the current confinement layer, and light is emitted there, then, light is emitted from the light-emitting exit of the p-side electrode as laser light while repeating reflection at the pair of multilayer film reflectors.
The above surface-emitting semiconductor layer generally has unevenness in which a polarization direction varies due to element variations and instability in which the polarization direction varies according to output and environmental temperature. Accordingly, when such surface-emitting semiconductor layer is applied to optical elements having polarized wave dependency such as a mirror and a beam splitter, for example, when the laser is used as a light source for the digital copy machine or the printer, variations in the polarization direction cause difference in a focus location or output of an image, which causes problem such as blur or color shading.
In response to the above problems, a technique of stabilizing the polarization direction into one direction by providing a polarization control function inside the surface-emitting semiconductor laser has been reported.
For example, in Japanese Patent No. 4138629 (Patent Document 1), it is disclosed that grooves having different depths are provided at two regions facing in the A-A direction as well as at two regions facing in the B-B direction orthogonal to the A-A direction around a resonator, and the number of oxidized layers in respective regions is changed to thereby increase compressive stress from one direction as well as increase polarized wave controllability.